Solder joints serve three functions: physical attachment of electronic components (for example peripheral packages) to a circuit board, completion of an electrically conductive path from the circuit component to the board, and provision of a thermally conductive path between the electronic component and the board. These functions may have conflicting requirements.
For physical attachment of electronic components it is typically the case that longer fatigue life may be achieved by use of larger solder joints. Such fatigue is caused by thermal expansion mismatch between the component and the circuit board. Similarly, larger solder joints typically improve thermal conductivity between component and board.
Use of smaller solder joints, whilst advantageous with respect to electrical conductivity for high frequency signals, has the disadvantage that resistance to solder joint fatigue failure may be reduced. It is also difficult to align small solder joints during re-flow in manufacture.
A known approach to improving package reliability with respect to solder joint fatigue is to underfill components with an epoxy resin (or other substance having similar adhesive, non-electrically-conductive properties) in order to constrain movement of a package relative to a board. This results in additional production processes--underfill dispensing and curing, either by heating or by ultra-violet irradiation--adding to time and cost of manufacture. The use of epoxy underfill also makes subsequent re-work more difficult and, in particular, precludes simple re-soldering.
It is known to provide strain relief for corner joints between an electronic component and a board by enlarging the solder joints at those corner joints.
U.S. Pat. No. 5,639,013 discloses a method for improving reliability of solder joints by forming convex solder joints having increased pedestal height.